The present invention relates to wastewater treatment systems and in particular to anaerobic reactors for treatment of wastewater.
Anaerobic processes have been used for several decades to treat wastewater in the United States. To anaerobically treat wastewater, the wastewater must first be exposed to bacteria. The bacteria convert organics in the wastewater into carbon dioxide and methane. This conversion is done in the absence of O.sub.2, NO.sub.3, and SO.sub.4. During this conversion, protons are transferred to carbon atoms to form methane.
Several bacteria are involved in the process. These bacteria can be divided into four groups. Hydrolytic or fermentative bacteria serve to convert complex sugars, proteins and fats to sugars, amino acids and fatty acids by the addition of water. Acetogenic bacteria convert sugars, amino acids and fatty acids to acetic acid and one-carbon compounds. Homoacetogenic bacteria synthesize acetic acid to form carbon dioxide and formate. The last group of bacteria, methanogenic bacteria, break down acetate to form methane and carbon dioxide. Of all the steps, the energy released is the smallest in the synthesis of methane.
During anaerobic digestion of organics within the wastewater to be treated, bacteria become attached to solid waste or other media suspended within the wastewater. As a result, bacteria may become clustered within distinct areas or pockets of the wastewater. Therefore, the wastewater is preferably mixed to maximize the volume of wastewater coming into contact and being digested and converted by the bacteria.
The time a system requires to convert organics and the system's efficiency for removing organics depend upon the number of bacteria in the wastewater during treatment. Moreover, because bacteria die without organic wastewater to digest, the bacterial count is also critical to the survival of the system during shutdown of the system. Methanogens, the final group of bacteria which convert acetate to methane and carbon dioxide, are the slowest multiplying and the most susceptible of all the bacteria. Thus, any anaerobic process must insure that the number of methanogens in the system is maintained. The number of methanogens in an anaerobic system depends on two factors: the number of methanogens washed out of the system with the effluent and the reproductive rate of the methanogens within the wastewater.
The first factor affecting the efficiency of an anaerobic reaction system is the loss of bacteria during discharge of treated wastewater or effluent from the system. After the wastewater has been sufficiently treated to contain acceptable levels of organic material, the treated wastewater or effluent is discharged for further treatment, use or disposal. However, removing the effluent from the system also removes many of the bacterial solids suspended in the wastewater to which the bacteria have attached themselves. Consequently, this loss of bacteria, otherwise known as insufficient solids retention, results in the loss of key microorganisms followed by poor treatment efficiency and eventual failure of the anaerobic wastewater treatment system.
In an attempt to prevent insufficient solids retention, conventional systems have employed reactors with large detention times, reactors with media, or reactors with intensive management of microbial populations to achieve the high solids retention required for treatment of wastewater.
Other municipal reactors or digestors have attempted to increase solids retention by the addition of a settling step in the process, or by addition of media while keeping the solids retention time and reactor volume as small as possible. For example, systems employing an anaerobic contact process use external settling in a clarifier to achieve solids retention. However, these systems are not efficient because transportation of the wastewater to an external settling clarifier alters the settling characteristics and increases the amount of solids suspended in the wastewater. Consequently, more time is needed to sufficiently settle out the bacterial solids.
Another system which uses settling for increasing the solids retention is an anaerobic sequencing batch reactor (ASBR). These reactors use internal settling to achieve a high efficiency of organic removal (80-90 percent). The reactor is also resilient to shut down and temperature changes. Despite these advantages, the ASBR is incapable of handling a continuous flow of wastewater. Instead, the wastewater must be treated in batches. As a result, a wastewater storage facility is normally required for holding influent while a batch reacts.
Other systems have attempted to achieve increased solids retention through the use of filters and sludge blankets. Systems using anaerobic filters require media to increase solids retention. These systems have a high efficiency of removal and are resilient to shut downs and temperature changes. However, the required media is expensive.
Systems employing an upflow anaerobic sludge blanket reactor use a floating blanket of sludge to remove bacterial solids from the effluent. Although this achieves a high efficiency of removal, such a system is difficult to manage.
The second factor affecting the efficiency of an anaerobic system is the reproductive rate of the bacteria. This reproductive rate, in turn, depends on the temperature of the wastewater. In order to maintain efficiency of the anaerobic reactor and to prevent its eventual failure, the bacteria must be given sufficient time to reproduce. This minimum required time is known as the minimum solids retention time (SRT). Generally, the minimum SRT is equal to the population doubling time of the slowest methanogens At 35.degree. C. the minimum solids retention time is around 10 days long. However, for every 10.degree. C. drop in the temperature, the minimum SRT is doubled. Thus, the minimum SRT is much longer in non-temperate, cooler climates. Consequently, anaerobic treatment systems in non-temperate climates generally require greater time to treat wastewater.
The present invention addresses these and other problems related to anaerobic reactors in a wastewater treatment system.